Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN  A CARRIER GAS OR AIR 
  • F23C9/00—Combustion apparatus characterised by arrangements for returning combustion products or flue gases to the combustion chamber
  • F23C9/006—Combustion apparatus characterised by arrangements for returning combustion products or flue gases to the combustion chamber the recirculation taking place in the combustion chamber
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23D—BURNERS
  • F23D1/00—Burners for combustion of pulverulent fuel

Definitions

• This invention relates to a method in accordance with the preamble of claim 1 for burning pulverized fuel in a tangentially fired boiler and for the production of reducing conditions to reduce nitrogen oxides.
• the invention also concerns an apparatus for implementing the method.
• a new type low-NO x burner has been constructed so that the air-deficient zone is formed very close to the tip end of the burner, and the two-stage combustion is carried out by means of a single-burner.
• This single burner staging technique combined with staging in the whole furnace (OFA, Over Fire Air technique) is very efficient in lowering NO x emissions.
• the United States patent No. 4,545,307 describes this kind of a low-NO x burner.
• the burner described in the US 4,545,307 is designed to be mounted perpendicularly in the wall of the furnace.
• These burners are equipped with a flame holder at the open end of the fuel feeding pipe, which promotes rapid ignition of the pulverized fuel; hence it is possible to allow a high temperature reducing zone to form near the burner.
• the flame holder is efficient also in reducing the amount of unburnt carbon, in addition to reducing NO x emissions.
• pulverized fuel is fed with carrier air, amounting to 20 to 30% of the total combustion air passed through a coal pipe and injected through an injection port and flame holder into a combustion furnace.
• a stream of secondary air having a swirling motion imparted by air vanes is passed through a secondary air register.
• a stream of tertiary air is passed through a tertiary air register, and it has a swirling motion imparted by a radial swirler.
• the primary combustion zone is separated from the secondary and tertiary air streams near the burner throat in order to form a good reducing atmosphere and, on the other hand, to enhance postflame mixing between unburnt carbon and tertiary air.
• a tangential jet burner of the prior art typically comprises a fuel pipe, secondary air channel and sometimes intermediate air channel for cooling materials between fuel pipe and secondary air channel.
• the distance between the ignition point and the throat of the bumer is 2 - 3 meters, and the burning of the fuel occurs mainly in the central vortex.
• the parallel streams of fuel and combustion air have been mixed together causing combustion in oxidizing atmosphere and forming NO x emissions.
• an air- deficient reduction zone does not form until the central vortex, and no staging occurs in the fuel stream between the throat of the burner and the central vortex.
• the staging concerns only the central vortex flame, and as deep staging as in the modem wall low-NO x burners can not be achieved by using jet burners.
• the NO x emissions of existing tangentially fired boilers can be reduced by modifying the boiler and burners and installing an over fire air system (OF A), instead of installation of totally new low-NO x burners. Normally this means that the combustion is delayed, and, as a consequence, the amount of unburnt carbon increases, and only a moderate NO x reduction can be achieved.
• OF A over fire air system
• This system inculdes a windbox and a first cluster of fuel nozzles mounted in the windbox operative for injecting clustered fuel into the furnace so as to create a first fuel-rich zone therewith, a second cluster of fuel nozzles mounted in the windbox operative for injecting clustered fuel into the furnace so as to create a second fuel-rich zone therewith, and an offset air nozzle mounted in the windbox operative for injecting offset air into the furnace and towards the walls of the furnace.
• the system also includes two sets of overfire air nozzles. With this system, fuel-rich zones are formed in the furnace and staged combustion occurs over the whole furnace. Emissions of nitrogen oxides are thereby reduced, but the system has several drawbacks. It is very complicated, and the furnace has requires quite extensive modifications.
• the object of this invention is to provide a totally new type of bumer and a combustion method for reducing the emissions of nitrogen oxides in tangentially fired boilers.
• a stream of secondary combustion air is passed around the flame formed of the fuel to provide a separating blanket of air around the flame, and a stream of tertiary combustion air is directed towards the water walls and horizontally away from the flame.
• a bumer according to this invention is called a NR-JET burner in the following.
• the main object and advantage of this invention is a substantial reduction of emissions of nitrogen oxides in flue gases.
• NO x emissions of tangentially fired boilers can be reduced at least to the same level as emissions of the modem wall fired boilers.
• the staging occurs both in a separate primary combustion zone in front of the bumer and further in the main vortex with the overfire-air. With this new combustion method, a much deeper staging of the combustion can be achieved than in the conventional tangentially fired boilers.
• the slagging problem of tangentially fired boilers is avoided by directing air to the waterwalls and thereby providing an oxidizing atmosphere near the walls.
• the amount of unburnt carbon is reduced because of rapid ignition of the fuel, and, at the same time, flame stability is improved.
• the construction of the NR-JET bumer is relatively simple. Accoring to the invention the main application of the NR-JET burner is retrofitting old tangentially fired boilers. When an old boiler is retrofitted with these burners, NO x emissions are reduced remarkably, and combustion efficiency is improved, too.
• This invention provides a totally new type of low-NO x bumer for the tangentially fired boilers, NR-JET bumer, applying some of the above mentioned principles used in wall fired low-NO x burners.
• staging occurs both in the primary combustion zone in front of the bumer and in the main vortex with OFA.
• pulverized fuel is injected by carrier air to the amount of about 20 to 30 % of the total combustion air into the combustion furnace.
• Around the fuel pipe there is concentrically a secondary air channel, for injecting the secondary air into the furnace.
• In uppermost and lowermost parts of the bumer there are tertiary air channels and representative injection ports.
• Fig. 1 is a schematic front and cross sectional view of the conventional Jet-bu er for tangentially fired boilers.
• Fig. 2 is a schematic front and cross sectional view of one embodiment of the invention.
• Fig. 3 is a schematic front and cross sectional view of the second embodiment of the invention.
• Fig. 5 is a schematic view of the fourth embodiment of the invention.
• _ ⁇ ⁇ is the volatilization zone, I the primary recirculation zone, II the reducing zone, HI the vigorous turbulent combustion zone, IV the tertiary recirculation zone, V the stagnation zone, VI the secondary recirculation zone and VII the main vortex.
• a conventional jet-bumer consists of rectangular pulverized coal pipe 1 and injection port 2 of the coal pipe.
• fuel pipe 1 there is upper secondary air channel 3 with upper secondary air injection port 4 and lower secondary air channel 5 with lower secondary air injection port 6.
• reducing zone ⁇ is very small.
• FIG. 2 shows NR-JET 1 burner according to the invention.
• the NR-JET comprises rectangular pipe 1 for pulverized fuel and injection port 2 in the outlet end of the fuel pipe.
• Around the fuel pipe there is concentrically arranged rectangular secondary air channel 7, forming a secondary air passageway around the outer periphery of pulverized fuel pipe 1, and injection port 8 for channel 7.
• NR- JET 1 bumer is also equipped with flame holder 9, which comprises ring 9a inside coal pipe 1 and guide sleeve 9b in secondary air channel 7.
• Ring 9a has the same rectangular form as the cross section of injection port 2 of fuel pipe 1, and it extends perpendicularly towards the central axis of fuel pipe 1.
• the cross section of ring 9a may be a continous ring, but in this construction ring 9a is provided with teeth, that extend into fuel pipe 1.
• Secondary air channel 7 surrounds the end part of coal pipe 1 and outward secondary air guide sleeve 9b of flame holder 9 extends into channel 7.
• the outer part of secondary air channel 7 of NR-JET 1 bumer is provided with angled guide sleeve 10.
• the vertical outward angle of angled guide sleeve 0 2 is typically between 5-40 ° in relation to the central axis of the bumer.
• Flame holder 9 is a ring that surrounds the inner wall of fuel pipe 1, and it is made of, or coated by, a wear and heat-resistant material such as ceramics or heat- resistant steel.
• flame holder 9 is a rectangular or cylindrical bluff body having a hole through which the pulverized coal stream is passed in the central part thereof, and it is arranged in the opening end of fuel pipe 1.
• the apron may be a continuous ring, but in this embodiment it is serrated, i.e. provided with cut-away parts in it.
• the inner diameter or dimension d t of ring 9a of flame holder 9 and inner diameter d 2 of fuel pipe 1 are preferably determined to satisfy a relation of 0.7 ⁇ ⁇ 0.98, and most preferably determined so as to give a dj/d 2 of about 0.9.
• the ratio of d-/d 2 is not limited to the above range, but if the ratio of d,/d 2 is too low, the flame holder protmdes too much into fuel pipe 1, increasing the flow rate of the pulverized coal stream passing through the injection port, and hence increasing the pressure drop inside the fuel feeding pipe.
• Angle 0 ⁇ formed between angled secondary air guide sleeve 9b and central axis of the fuel pipe is typically between 15 - 25 ° in order to give enough flame maintenance effect and to separate well the central reducing flame from the oxidizing main flame and the combustion air.
• NR-JET 2 bumer comprises rectangular fuel pipe 1 having injection port 2.
• rectangular secondary air channel 7 forming a secondary air passageway around the outer periphery of fuel pipe 1, and injection port 8 of channel 7.
• In the uppermost and lowermost parts of the bumer there is upper tertiary air channel 11 and lower tertiary air channel 13, and corresponding injection ports 12 and 14.
• the primary function of the spacers is to separate the secondary and the tertiary air streams in order to protect the formation of reduction zone II in front of the bumer.
• the height (d 3 ) of spacers 15, 16 varies normally between 30 and 350 mm.
• Flame holder 9 is similar to that in NR-JET 1 bumer.
• Both upper and lower tertiary air channels 11 and 13 are also equipped with guide sleeves 17 and 18 having vertical angle 0 3 . Normally 0 3 is between 5 and 40 °.
• the length of the sleeves should be designed so that the relation between the length of sleeve 1 and the height of the tertiary air passage h j is l/h, >2 (figure 3.).
• Tangential jet bumer NR-JET 3 is basically similar to NR-JET 2 bumer, with the exception of air vanes 19 being provided in the passageway of secondary air channel 7. These axial vanes 19 give the secondary air stream a tangential velocity component improving the turbulent combustion near the bumer throat. Typically the number of air vanes 19 is 8 - 15, and the vanes are angled to the axial direction in an angle of 40 - 50° so that swirl number is between 0.5 and 1.0. Another difference between NR-JET 2 and NR-JET 3 is the shape of the fuel pipe and the air channels.
• Fuel pipe 1, fuel injection port 2, secondary air channel 7 and secondary air injection port 8 have a cylindrical shape and, as earlier, they are equipped with flame holder 9, which comprises angled secondary air guide sleeve 9b, and toothed ring 9a.
• Flame holder 9, spacers 15, 16, tertiary air channels 11, 13 and tertiary air injection ports 12, 14 are cylindrically shaped.
• the amount of primary air depends basically on the mill conditions, being typically between 20 and 30 %. Favourable velocity of the primary air is 15 - 25 m/s.
• one object of the secondary air is to prevent spreading of the coal/primary air stream.
• the secondary air is passed around the reducing flame ⁇ with a great velocity, and it forms a separating blanket reducing the amount of coal particles that are driven to the furnace walls, and the slagging behavior of the boiler is reduced.
• the amount of primary and secondary air should enable the burning of the volatile material of the fuel.
• the precentage of volatiles in the coal or other fuels determines the amount of the secondary air, being normally less than 30%.
• the velocity of the secondary air has to be sufficiently high, about 30 - 80 m/s.
• the rest of the combustion air is injected through the tertiary air injection ports, and the mass flow ratio between secondary and tertiary air is 1:2-1:5.
• the velocity of the tertiary air at the tertiary air injection port is 30-80 m/s. If the content of volatiles in the fuel is low, the amount of primary air may be sufficient for combusting these volatiles in the reducing flame. In such case, mixing of secondary air with the reducing flame must be prevented.
• the secondary air flow is similar to the tertiary air flow, and no separate secondary air streams exists unlike in NR-JET 2 and 3 burners.
• the combustion air channel may surround the primary air/fuel channel, or it may be arranged in two separate channels above and below the fuel pipe.
• Another very important fact concerns flame stabilization and mixing: in case of swirling burners, tertiary air has high swirl number that gives good postflame mixing and stabilization. In case of tangential NR-JET bumer, tertiary air has only axial momentum, but in this case central vortex compensates the lack of the swirl and takes care of mixing and flame stabilization
• NR-JET 1 burner is equipped with flame holder 9, which enhances the formation of primary recirculation zone I improving ignition and flame stability.
• the secondary air is passed around the primary air and fuel with a great velocity, and this prevents spreading of the fuel stream.
• the passage for secondary air 8 is shaped to direct a part of the secondary air away (ring 9a + sleeve 9b, ⁇ from the primary air and fuel. As a result, reducing zone II is larger and nearer the bumer throat than in the conventional jet burner.
• NR-JET 2 burner is equipped with flame holder 9, which enhances the formation of primary recirculation zone I improving ignition and flame stability.
• the ignition and flame stability of NR-JET 2 bumer is improved compared to NR-JET 1 bumer thanks to the tertiary recirculation zone IV.
• This is a consequence of the underpressure zone formed between secondary and tertiary air streams, whereby hot flue gases from the main vortex are recirculated back to the combustion zone.
• less secondary air is mixed into the volatilization zone avoiding the dilution effect and enhancing the ignition and flame stability, compared to NR-JET 1 bumer.
• tertiary air injection ports 12 and 14 are shaped to direct the tertiary air away from the center of the fumace and towards water walls 23 of the fumace (figure 6).
• oxygen in kept away from the centre of the fiimace and near water walls 23 so as to prevent reducing atmosphere to form there.
• the slagging of the lower fumace is also reduced, and the lower fumace heat absorption is increased.
• Angle 0 7 between tertiary air flow 26 and wall 23 is preferably 5-45°, and the guide sleeves in the tertiary air passages are arraged accordingly.
• Figure 6 also shows fuel flow 25 from the comer of the fumace to central vortex 24, where the fuel finally bums.
• venturi part 20 In case of NR-JET 1,2 and 3 burner, it is possible to apply inside fuel pipe 1 venturi part 20 and pulverized fuel concentrator (P.F. concentrator) part 22.
• P.F. concentrator pulverized fuel concentrator
• This kind of fuel pipe is shown in figure 5.
• Venturi 20 is located at a distance from the exit end of fuel pipe 1, and the concentrator extends through the throat of venturi 20.
• the dimensions of concentrator 22 start to enlarge at the same time as the inner diameter of fuel pipe 1 starts to enlarge after venturi 20.
• the dimensions of concentrator 22 start to diminish near the exit of pipe 1, and concentrator 22 ends in the vicinity of flame holder 9. With venturi 20 it is possible to achieve more uniform fuel particle distribution before P.F. concentrator 22.
• the fuel concentrator is arranged on the centerline of the fuel pipe and has a bulge part forming an angle of 5 - 60° (0j) at the leading side of the fuel stream, and an angle of 5 - 30° (0 6 )at the exit side of the fuel stream.

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• 1993
  • 1993-11-08 AU AU54225/94A patent/AU5422594A/en not_active Abandoned
  • 1993-11-08 DE DE4395243T patent/DE4395243T1/de not_active Withdrawn
  • 1993-11-08 US US08/637,777 patent/US5799594A/en not_active Expired - Fee Related
  • 1993-11-08 CZ CZ19961302A patent/CZ290627B6/cs not_active IP Right Cessation
  • 1993-11-08 HU HU9601208A patent/HU220143B/hu not_active IP Right Cessation
  • 1993-11-08 RU RU94045853A patent/RU2104443C1/ru active
  • 1993-11-08 WO PCT/FI1993/000461 patent/WO1995013502A1/en active IP Right Grant
• 1994
  • 1994-11-08 PL PL94305749A patent/PL185958B1/pl unknown
  • 1994-11-08 CN CN94120063.9A patent/CN1095970C/zh not_active Expired - Fee Related

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